Analgesics are often prescribed to relieve post-operative pain. The great difficulty in properly administering analgesics stems from a variety of factors. Age, hypatic function, renal function and other medication all affect the pharmacokinetics of analgesics and greatly affect the patient's need for analgesics. Thus while some patients never request (and thus do not need) analgesics for pain relief, some patients continue to suffer even after conventional doses of analgesics have been administered. Further, doctors tend to underprescribe the use of analgesics and nurses tend to underadminister them because of the fear that the patient will become addicted to the analgesic.
In the last several years, there has been considerable activity directed to devices and systems which permit the patient to control how much analgesic he or she receives up to a maximum predetermined limit. It has been found that as a group, patients controlling the quantity of analgesic they receive use less analgesic than patients who request the administration of an analgesic. Apparently, one factor is the psychological relief present when a patient knows he or she is in control of the amount of drug to be received, up to a maximum limit.
Devices that are on the market, or that are in the process of obtaining government regulatory approval, that are directed to the patient-controlled delivery of analgesics, include the Cardiff Palliator by Pye Dynamics Ltd. or Graseby Dynamics of the United Kingdom; the On-Demand Analgesic Computer (ODAC) Model JSI 0299 made by Janssen Scientific Instruments; a PCA infuser by Abbott Laboratories, Inc.; the Harvard PCA Pump by C. R. Bard Inc.; and a pump by Deltec Systems Inc. All of these pumps are large and bulky, the smallest pump being the Deltec pump, which is approximately as large as a telephone. All of the above-mentioned devices are electromechanical in nature, requiring a separate power source. Although the Deltec unit may conceivably be worn by patients, it is believed that the remainder of the pumps mentioned above confine the patient to a bed, or some other fixed location.
Another problem associated with these devices is that after the drug is loaded into the pump, certain control factors must be set by the nurse or other person who actually sets up the pump with the patient. Yet another problem with existing devices is that they are relatively expensive and may include some rather complex electronic components.
Another apparatus and system for patient-controlled analgesic is described in PCT International Publication Number 87/00758. The apparatus includes a dose reservoir for receiving and storing a dose of the analgesic, an inlet and an outlet to the dose reservoir, and control means operative by the patient for selectively expressing beneficial agent out of the dose reservoir through the outlet means. The inlet to the dose reservoir receives the analgesic under pressure from a pump means separate from the patient-controlled analgesic device itself which pumps the analgesic to the apparatus from an external supply source.
Known patient-controlled analgesic systems have several drawbacks. They are generally very complex and difficult to use. Often times they must be calibrated prior to use and this typically requires the user to be extensively trained in the use of the device. Also an auxiliary pumping source is typically needed to feed beneficial agent to the patient-controlled analgesic apparatus. In addition, such systems are comparatively expensive. Further, because of their size, many of the known patient-controlled analgesic systems are not suitable for ambulatory use. Thus there remains a need for an accurate, self-driven, low cost, patient-controlled analgesic apparatus, and in particular, such an apparatus that is adaptable for ambulatory use.
Accordingly it is a principal object of the present invention to provide a patient-controlled analgesic device which is self-driven. A related object is to provide such a device in which the device itself includes a single power source for both filling the device with beneficial agent and for discharging up to a maximum predetermined dose of the beneficial agent to the patient upon demand of the patient. It is a further related object to provide such a device wherein the power source has a linear fill rate over its entire fill range in order to prevent overdosing.
It is yet another object of the present invention to provide a patient-controlled analgesic device that can be used with prefilled containers of the beneficial agent to be administered to the patient. It is also an object of the invention to provide a patient-controlled analgesic device that is suitable for ambulatory use.
These and other objects and advantages of the present invention will become apparent from the following detailed description of the invention and from the accompanying drawings.